THE TRANSPORTATION OF GASES 167 FIG. 25.—Taylor hydraulic compressor in-near Cobalt, Ont. through suitable gates to two "heads" each 16 ft. in diameter and containing 66 pipes 14 in. in diameter. The size of the heads is reduced in diameter to about 8 ft. and the whole apparatus can be raised or lowered as required by operating conditions. A cone a assists in separating the air and water, and the long horizontal tunnel permits quite complete separation. The compressed air is removed through the pipe c and the water freed from the entrained air escapes through the vertical shaft b. Pipe e acts as a relief for a surplus of compressed air. Its end is normally below the surface of the water in the tunnel, but if too much air should accumulate the end would be exposed and permit the escape of the surplus without seriously affecting the normal air pressure of the distribution system. Piston Compressors and Blowers.—The large quantities of air required for blast-furnaces and Bessemer converters are usually supplied by piston compressors of large capacity, driven either by steam or gas engines. Turbo-blowers directly driven by steam turbines, however, have been recently developed for this work. Piston compressors used as blowers for blast furnaces and Bessemer converters usually operate at discharge pressures of 15 or 20 Ib. gage. But small compressors of this type are built for pressures as low as 1 Ib. above atmosphere, single-stage up to 80 to 100 Ib., two-stage from 80 to 500 Ib., three-stage to about 1,200 Ib. (small units even to 2,500 Ib.) and four-stage to 3,000 Ib. or even higher. The tendency in blower design is to secure increased capacity by higher speeds than formerly. The Allis-Chalmers Company now uses a maximum speed of 90 r.p.m. for 48-in. stroke and 85 r.p.m. for 60-in. stroke, giving piston speeds of from 720 to 850 ft. per minute. With piston speeds approximating 750 ft. per minute the inlet area is approximately 13 per cent and the outlet area 11 per cent of the piston area. With increased speeds the tendency is to increase clearance, which will usually vary 9J^ to llj^ per cent. The Slick blowing tub consists of a reciprocating cylinder on the outside of the compressing cylinder, arranged so as to open ports at the ends of the cylinders for inlet. This gives unobstructed inlet areas of from 18 to 20 per cent with very small clearance. The ratio of inlet valve area to piston area varies from 0.05 to 0.14. For ordinary types of valves, the inlet area should, as a rule, be not less than 8 or 10 per cent, of the piston area. Automatically operated inlet valves are apt to be irregular in their action and reduce the volumetric efficiency of the compressor. This disadvantage is overcome by using mechanically operated inlet valves, usually of a Corliss type, but on account of the variation of discharge pressure they are not used so often for discharge or for inlet on the high-pressure stages of a multi-stage compressor. The area of the discharge valves will usually vary from 10 to 15 per cent of the piston area, the larger percentage being required for the higher piston speeds. . In the Laidlaw-Dunn-Gordon compressors a valve gear is used that mechanically controls the openings and closing of the suction and the closing of the discharge.